An electrical power converter is a device for processing electrical power from one form into another form that meets the requirements of an electrical system. Electrical power converters commonly are used to change alternating-current (AC) power to direct current (DC) power or vice versa, or to change one DC voltage level to another. A power converter that changes DC power to AC power is commonly referred to as an "inverter." A power converter that changes AC power to DC power is commonly referred to as a "rectifier." A variety of circuit topologies are used as the basis for power converters, including the well-known topologies referred to as buck, boost, buck-boost, push-pull, bridge, half-bridge and forward converters.
In many power converters, a switching regulator regulates the output signal of the converter by varying the amount of time that electrical energy is coupled from the converter's input to its output. To accomplish this function, the switching regulator controls one or more switches that govern the coupling of voltage and current from the input of the converter to key circuit elements within the power converter. The regulation process commonly is performed by a pulse width modulator that is responsive to the output voltage of the power converter. The pulse width modulator controls the amount of time the switches are ON (i.e., power is enabled to couple through the switches), thereby determining the pulse width of the current and voltage pulses coupled from the input of the converter. The pulse width, or length, of the input current and voltage pulses in turn affects the characteristics of the output voltage.
The characteristics of the output voltage waveform of a power converter are often important to the performance of the load device and to meet standard specifications. For general purpose power inverters, for example, a near-sinusoidal waveform with a total RMS harmonic content of not more than 5% of the fundamental value is often specified, as well as a 3% limit on any individual harmonic.
The ability of an inverter to maintain the appropriate output voltage waveform is, however, limited by the impedance characteristics of the load device. For example, when the power inverter is designed to produce a sinusoidal output voltage, inductive loads commonly cause harmonic distortion of the output voltage waveform. This effect occurs because power converters and inverters commonly have a capacitance at their output. Inductive loads cause the inverter's output current waveform to lag its voltage waveform so that current is coupled from the load to the inverter during a positive voltage half-cycle. The current returned from the load raises the voltage at the inverter's output capacitance, resulting in harmonic distortion of the output voltage waveform.
Similarly, resistive loads that are light (i.e., that draw little power in comparison to the power delivery capability of the inverter) can also cause distortion of the output voltage waveform. In such cases, the load does not draw sufficient power for the capacitor to discharge at an adequate rate when the reference waveform is decreasing. Thus, the decrease in the output voltage lags the decrease in the reference waveform, resulting in harmonic distortion of the output voltage waveform.
Accordingly, there is a need for an electrical power inverter for providing power to a load with an output voltage waveform that is less sensitive to the characteristics of the load.